Page 267 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:38 PM Page 229

Heat Transfer 229

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Total tubing length: A o 0.678 ft /ft Enthalpy of liquid 26.28 Btu/lb
U L U o (A o , finned tubes) (75.5) (0.688, updated Table 10-39) At 30°F and 28.46 psig evaporation condition
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51.9 Btu/(hr) (°F)(ft ) tubing) Enthalpy of the vapor 81.61 Btu/lb
Enthalpy change 81.61 28.46 53.15 Btu/lb
Q 3,935,000 Freon flow 3,935,000/53.15 74,035 lb/hr
L t 2,445 ft finned tubing At 80°F and 84 psig, liquid 0.0123 ft /lb
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U L 1 t o 2 151.921312
Water flow (187 gpm) (8.34 lb/gal) 1559.6 lb/min 74,03510.0123217.482
gpm 113.5
1602
12. Number of tubes to give 5 ft/sec water velocity
Design gpm (113.5)(1.25) 141.8
Use 3-in. nozzle, velocity 6.2 ft/sec
1187 gpm210.002882

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2
2
10.519 in >tube I.D.211>144 in >ft 213.9 ft>sec2
Vapor Freon-12 Out:
30.3 tubes>pass, use 30 tubes
From Figure 10-63, max. allowable vapor
Number of 12-ft tubes required, total 2,445/12 Velocity (28)(1.3) 36 ft/sec
203.9, use 204 tubes To reduce entrainment, use 25–30 ft/sec
Number of tube passes 203/30 6.76 passes, use 6 passes At 30°F, vapor 0.939 ft /lb
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Q 3,935,000 174,035210.9392
L T 2445 ft Total flow 19.31 ft >sec
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151.221312
U L t o 13,6002
13. Number of tubes: For 12-in. nozzle, area of cross-section 0.777 ft 2
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Capacity at 25-ft/sec (0.777)(25) 19.5 ft /sec
1187210.8132
Tube velocity 5 This nozzle O.K.
No. tubes>pass
No. tubes/pass 30.4, say 30
Double Pipe Finned Tube Heat Exchangers
Number of 12-ft tubes, total 2,470/12 206
Number tube passes 206/30 6.85, use 6 passes
To properly rate and design this type of unit, the process
data should be submitted to the manufacturer, because ade-
14. For 6 passes:
quate published correlation literature is not available.
Figures 10-4A, 10-4B, 10-4C, and 10-4D illustrate the usual
Use 226 tubes on 1 / 4 -in. triangular pitch
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construction of finned-tube heat exchangers with the fins
Shell I.D. 25 in. (Table 10-9)
running parallel to the length of the tube. These are usually,
No. tubes/pass 226/6 37.7
but not always, installed with a tube or pipe outer shell. Typ-
Use: Passes of 38-38-38-38-37-37 tubes
ical fins are shown in Figure 10-152. Tube may be fabricated
Use shell diameter larger in order to have vapor disengaging
space; a diameter of 29 in. or 31 in. I.D. will be satisfactory—the with fins attached by resistance welding rather than imbed-
latter being the better choice. ding in the tube as shown in Figure 10-152B. The I.D. of the
3
1
internal finned pipe usually ranges from / 4 –1 / 2 in., and
1
Outside surface area, net (finned): the outside surrounding pipe shell can be 2 / 2 in., 3 in., and
1
3 / 2 in. nominal standard pipe size. The number of fins
3
1
1226210.6882112 ft 4 in.>122 1,814 ft 2 range from 18 for the / 4 -in. pipe, 24 or 32 for the 1 / 2 -in.
1
1
pipe, and 16 or 32 for the 1 / 2 -in. pipe with / 2 -in. fin
3,935,000
2
Actual U 69.9 Btu>hr 1ft 21°F2 height, per manufacturer Griscom-Russell/Ecolaire Corp.
131211,8142
The fins of Figure 10-152B are imbedded longitudinally in
groves “plowed” into the tube’s outer surface. The displaced
Nozzles for water inlet and outlet:
metal is squeezed back against the imbedded fin base to
Flow 187 gpm form a tight metal–metal bond. This bond is not affected by
Use 4-in. nozzle, velocity 4.79 ft/sec changes in temperature.
(Cameron Table, Fluid Flow Chapter) Except for fluid conditions of possible galvanic corrosion,
Liquid Freon-12 in: the fins can be any selected material, not necessarily the
80°F and 84 psig (from condenser) same as the tube. Some usable fin and/or tube materials are